1. Introduction
This invention relates to immersion solder deposition and more particularly, to pretreatment of a substrate prior to immersion solder deposition to provide uniform deposits from a solder depositing solution over the useful life of the deposition solution. The invention is especially useful for the manufacture of printed circuit boards.
2. Description of the Prior Art
Tin-lead alloy having a composition of between 60 and 65 percent tin and 35 to 40 percent lead (solder) is coated onto copper traces in the manufacture of printed circuit boards. The alloy may function as an etch resist and permits attachment of components to a circuit board. For this latter use, the alloy must melt and reflow at a reasonably low temperature to avoid damage to the circuit during heating of the board and must have an adequate thickness for surface mounting of components.
Conceptually, there are four methods available for coating tin-lead alloys onto copper traces in printed circuit manufacture. These methods include (1) hot air leveling, (2) electroless plating, (3) immersion plating and (4) electroplating. Each process has advantages and disadvantages and the metal deposit obtained from each differs from the deposit obtained by other plating methods. The invention described herein is a process sequence for immersion deposition of tin-lead alloys.
Immersion plating is a process where deposition is by displacement of elemental metal from a substrate by metal ions in a plating solution. Since plating is by displacement, immersion plating is an electrochemical reaction that depends upon the position the substrate metal occupies in the electromotive series relative to the metal to be deposited from solution. Plating occurs when the metal from a dissolved metal salt is displaced by a more active (less noble) metal immersed in the solution. Since copper is more noble than tin or lead, copper would not be expected to be plated by an immersion process to give a tin-lead deposit. However, when complexed under acidic conditions, the electropotentials of the tin and lead complexes relative to copper reverse making immersion plating possible.
Limitations in the use of immersion plating for circuit fabrication exist. These include a slow plating rate, difficulty in obtaining a desired alloy, limited deposit thickness, changes in deposit properties over an extended use of the plating solution and difficulty in obtaining a deposit suitable for reflow. Limited thickness is due to the plating reaction being self-limiting because as the coating builds, the metal deposited from solution masks the underlying base metal which functions as the reducing agent and is required for displacement. Additionally, as the displaced base metal is dissolved in solution, it becomes a contaminant progressively slowing the rate of displacement. Typical deposit thickness for an immersion tin deposit in the prior art is 50 to 100 microinches, mainly because of the foregoing problems. Further, as immersion deposition continues, the concentration of components in the solution changes through reaction, plate-out of metals, "drag in" of solution contaminants and drag out of solution components. Since the immersion deposition solution must be carefully controlled to obtain uniform deposits, small changes in solution composition can result in major changes in deposit properties.
Absent the above disadvantages, there would be advantages to immersion plating compared to other methods for depositing tin-lead alloy in circuit manufacture. Compared to electroless and electroplating, there is no hydrogen generation during the plating process and no concomitant pitting or other discontinuities in the deposit. Also, immersion plating is not subject to surface roughness as found in electroplating processes due to "drag-over" from precleaners, anode corrosion and the like. Further, since electroless baths contain both a metal to be plated and a reducing agent, the bath is potentially unstable subject to spontaneous plating. In immersion plating, there is no problem with spontaneous plating. Moreover, with immersion plating, neither an electrically continuous circuit nor attachments of electrical contacts are required nor is there a need to maintain a precise current. Finally, an immersion deposit is generally uniform in thickness.
It is known in the art that to be suitable for printed circuit board manufacture, a tin-lead alloy deposit over copper circuit lines must be pore free, reasonably thick, should possess a uniform cross section and should be bondable to subsequently attached components. It is also known that as metal is deposited onto a substrate from solution in a plating process, the deposit obtained is often not coherent and may contain numerous pores due to the nature of the plating reaction. Coherency is obtained and pores are eliminated in a tin-lead alloy deposit by heating the board with the alloy deposit to a temperature in excess of the alloy's melting point whereby the deposit melts and pores are eliminated by reassembly of the deposit, a process known in the art as "reflow". To reflow a tin-lead alloy, the tin-lead alloy should melt at a relatively low temperature to avoid damage to the circuit by heating to an excessively high temperature and the deposit must be adequately thick. A thin deposit will not reflow and may not contain the minimum mass of metal required to bond a component to a circuit board, particularly if the component is surface mounted on the board. To obtain a low melting tin-lead alloy, the deposit desirably has a concentration of tin and lead at or close to the low melting eutectic of tin and lead (approximately 63% tin and 37% lead by weight--i.e., solder).
In spite of the potential advantages of immersion plating, the prior art generally dismissed immersion plating processes for printed circuit fabrication because the prior art believed that thick, bondable (solderable) deposits capable of reflow could not be obtained. As stated in Printed and Integrated Circuitry, McGraw Hill Book Company, Inc., New York, 1963 at page 138, (immersion deposits are) "limited in thickness, porous, and often poorly adherent and, therefore, of limited interest". The self-limiting feature and thickness of immersion tin and lead plating procedures were believed to make soldering impossible and consequently, plating baths for immersion deposits of tin and lead were of minimal interest to the art.
In U.S. Pat. No. 4,194,913, incorporated herein by reference an immersion plating composition is disclosed for deposition of tin-lead alloys. In accordance with the teachings of this patent, an immersion plating solution is disclosed comprising stannous chloride, lead chloride, sodium hypophosphite, thiourea, hydrochloric acid and gelatin. In this patent, patentee states that the plating composition provides a faster plating rate and a deposit of increased thickness. Though it is believed that improved immersion deposits are obtained using the compositions of the patent, it is believed that the deposits are unsuitable for the commercial manufacture of printed circuit boards.
In European published application No. 0 167 949, also incorporated herein by reference, an immersion plating solution is disclosed that may contain a tin and lead salt of a fluorine containing mineral acid, a fluorine containing mineral acid sufficient to provide a pH up to 0 and 1, and a sulfur containing complexing agent such as thiourea. An example of a tin-lead plating solution is given though it is believed that thick, reflowable deposits of tin and lead are not obtained and the compositions are believed to be unsuitable for the circuit fabrication.
In copending U.S. Pat. Application Ser, No. 07/532,819 filed Jun. 4, 1990, now abandoned, assigned to the assignee hereof and incorporated herein by reference, an immersion plating solution capable of plating a thick, porous, adherent tin-lead alloy deposit capable of low temperature reflow is disclosed. The invention of said application was based on a combination of discoveries. For example, it was found for deposit reflow, the deposit had to be thick and porous--i.e., at least 100 microinches and preferably 150 microinches or greater. Also, to obtain a thick deposit, the plating solution should favor a porous structure--not a dense deposit. To obtain such a deposit, the solution should contain a high metal content--i.e., in excess of 0.10 moles per liter with a tin to lead ratio of at least 1 to 1. Further, the tin salt, the lead salt, and the acid used to adjust pH should desirably contain a fluorine anion. Recognizing that the displacement reaction favored tin, a lead promoter was added to solution to maintain concentration of lead relative to tin. It was also found that dissolved copper was a desired component to promote plating rate. Finally, exaltants are added to solution.
Following deposition of a tin-lead alloy from the immersion plating solution described above, the deposit formed had a crystalline porous structure with alternating layers of lead and tin and was capable of reflow by heating to a temperature above the deposit melting point for a time sufficient to form a dense coherent coating. After reflow, the alloy was dense and shiny and believed to be a true homogeneous alloy of tin and lead.
The immersion deposition composition of the copending application provided solder alloys suitable for the manufacture of printed circuit boards. However, as should be evident from the discussion set forth above, the solution required operation within restricted concentrations and conditions to obtain deposits suitable for use in circuit manufacture and suitable for reflow. Variations in the conditions or composition of the immersion depositing solution resulted in changes in the deposit obtained and consequently, lack of uniformity from deposit to deposit during use of the plating solution over a prolonged time.